GENUS POLYSTOMELLA : INTERNAL STRTJCTTJRE. 543 



towards the circumference. The conical hollow thus left on each side in the central 

 portion of the shell, is entirely filled up by the solid nucleus already adverted to : the 

 calcareous deposit, however, of which the nucleus is composed is by no means limited 

 to it, but extends over the whole outer surface of each whorl, except where (in well- 

 preserved specimens) the portion last formed is as yet unconsolidated by it. For a 

 careful examination of sections taken in different directions, makes it clear that whilst 

 the internal portion of the spiral lamina that forms the outer wall of each chamber is 

 continuous with the nearest lamella of the adjacent septum on either side (Plate XVII. 

 fig. 10), the substance of the external portion is no less continuous with that of the 

 calcareous nucleus. The additional deposit is obviously homologous with that which 

 forms the " intermediate skeleton " in Cycloclypeus (^[ 99), though less differentiated 

 from the proper walls of the chambers than we have seen it to be in that type, or than 

 we shall find it to be in Calcarina (^[202). The whole thickness of the spiral lamina 

 is generally traversed by minvite tubuli, passing in a radial direction from one surface 

 towards the other ; but these have by no means either the closeness or the regularity 

 which distinguishes the tubular structure in Cycloclypeus and Operculina, and the shell- 

 substance is in many parts so destitute of tubuli as to be of almost glassy transparence. 

 The fun-owing of the external surface (^[ 180) is seen in vertical sections (Plate XVII. 

 fig. 2) not to be produced by mere superficial excavations, but to proceed from a plicated 

 arrangement of the spiral lamina ; and this is related to the prolongation of the posterior 

 margin of each segment into a series of "retral processes" (Plate XVIII. fig. 12, a), 

 corresponding to those described by Professor WILLIAMSON in Polystomella crispa 

 (^[ 175). They are, however, much less elongated in this type, simply giving a crenu- 

 lated margin to that angle of the segment, which contrasts remarkably with the smooth 

 unbroken aspect of its anterior border #, V. The spiral lamina which forms the outer 

 wall of the chamber, being modelled (so to speak) upon the surface of these retral pro- 

 cesses, presents internally a corresponding series of grooves, which are deepest towards the 

 posterior margin, and become rapidly shallower in passing towards the anterior margin, 

 of each chamber, as is shown at , a 1 , 3 , figs. 1 and 11, Plate XVIII. : these grooves 

 are not, however, as in P. crispa, completed into tubes for part of their length by an 

 additional lamella of shell given off from the septum (^f 175); but they are sometimes 

 shown, in sections which happen to traverse them (Plate XVII. fig. 10), to be extended 

 into csecal prolongations (#, a) by backward inflexions of the septa at their junction with 

 the spiral lamina. 



186. The communication between the successive segments of the same whorl is esta- 

 blished by a number of minute processes or stolons of sarcode (c, figs. 12, 13, Plate 

 XVIII.), which pass at regular intervals between their internal margins through a series 

 of pores which can be distinguished along the inner border of each septum (fig. 1, c, c', c") 

 close to its junction with the preceding convolution. I have not detected in any instance, 

 either in sections of the shell, or in the siliceous casts which so exactly represent the 

 sarcode-body, any other communications between the chambers or their contained seg- 



